Cardiomyocytes - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 205

thick myosin

actin

thin

filament

Z disc M line Z disc

H zone

I band I band

A band

Fig. 5.2 The sarcomere, its microscopic structure and the myofilament arrangement. The M line
and the Z discs are proteic scaffolds that orientate myosin and actin filaments, respectively. The
H zone (bisected by M line) contains only thick myosin filaments, the I band (bisected by Z disc)
only thin actin filaments, and the region of the A band close to the Z disc both filament types.

The sarcomere includes 4 filament complexes: (1) thin filament that is
predominantly made of F actin, 27 (2) thick filament, mainly consisting of myosin 28
(3) titin, 29 and (4) nebulin. The lateral boundaries of the sarcomeric units, Z discs,
relate contractile apparatus to cytoskeleton. Sarcomere shortening of striated
myocyte results from actin and myosin filaments sliding over each other, as the
lengths of these filaments remain constant.
Sarcomere Z discs that anchor myofilaments and stabilize their assembly, are
densely packed cellular structures that include actin, titin, and nebulin, as well as
smaller proteins, such as

-actinin and telethonin. Two giant titins are assembled
into a complex by telethonin , a Z-disc ligand that has a symmetric structure [ 358 ].

α

27 Striated myocytes express 2 actin isoforms, cardiac actin and skeletal actin. In adult myocardium,
cardiac actin is the major isoform (80%) [ 357 ].
28 There are three isoforms of non-muscle myosin, myosin-2A, myosin-2B, and myosin-2C.
Myosin-2 is also observed in muscles. Myosin-2B is the single isoform found in adult cardiomy-
ocytes.
29 Titin is also named connectin.